Method of producing oxidation catalyst for cleaning exhaust gas

ABSTRACT

The present invention provides a method of producing an oxidation catalyst for cleaning exhaust gas, capable of achieving an excellent catalytic activity at a lower temperature for particulates and high boiling point hydrocarbons in exhaust gas from internal-combustion engines. A primary firing is performed after mixing nitrate of a first metal element Ln, manganese nitrate, and oxide of a third metal element A. A resultant material from the primary firing is subjected to grinding and then a secondary firing is performed at the range of 600 to 1200° C. for 1 to 5 hours. By doing so, a catalyst comprising a composite metal oxide represented by the general formula Ln y Mn 1-x A x O 3  is obtained.

This application is a Continuation-In-Part of copending application Ser.No. 11/730,858 filed on Apr. 4, 2007, the entire contents of which arehereby incorporated by reference and for which priority is claimed under35 U.S.C. §120 and to Patent Application No. 2006-104269 filed in Japanon Apr. 5, 2006, the entire contents of which are hereby incorporated byreference and for which priority is claimed under 35 U.S.C §119.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing an oxidationcatalyst for cleaning exhaust gas, oxidizing particulates andhydrocarbons contained in exhaust gas from internal-combustion enginesto clean the gas.

2. Description of the Related Art

For oxidizing particulates and hydrocarbons contained in exhaust gasfrom internal-combustion engines to clean the gas, oxidation catalystscomprising perovskite-type composite metal oxides have been previouslyknown.

As the perovskite-type composite metal oxide used as the above-describedoxidation catalyst, there is known, for example, a composite metal oxiderepresented by the general formula: AB_(1-x)C_(x)O₃, wherein A is atleast one metal selected from the group consisting of La, Sr, Ce, Ba,and Ca; B is at least one metal selected from the group consisting ofCo, Fe, Ni, Cr, Mn, and Mg; and C is one of Pt and Pd (see JapanesePatent Laid-Open No. 07-116519).

As the perovskite-type composite metal oxide used as the above-describedoxidation catalyst, there is also known, for example, a composite metaloxide represented by the general formula: Ce_(x)M_(1-x)ZrO₃, wherein Mis at least one metal selected from the group consisting of La, Sm, Nd,Gd, Sc, and Y; and 0.1≦x≦20 (see, for example, Japanese Patent Laid-OpenNo. 2003-334443).

However, the above-described conventional perovskite-type compositemetal oxides have drawbacks that they have high oxidation temperaturesfor particulates and high boiling point hydrocarbons and cannot achievesufficient catalytic activities.

SUMMARY OF THE INVENTION

The object of the present invention is to eliminate such drawbacks andto provide a method of producing an oxidation catalyst for cleaningexhaust gas, capable of achieving an excellent catalytic activity at alower temperature for particulates and high boiling point hydrocarbonsin exhaust gas from internal-combustion engines.

For accomplishing this object, the present invention provides a methodof producing an oxidation catalyst for cleaning exhaust gas, whichoxidizes contents in exhaust gas from internal-combustion engines toclean the gas, and which comprises a composite metal oxide including afirst metal element Ln, Mn as a second metal element, and a third metalelement A, represented by the general formula Ln_(y)Mn_(1-x)A_(x)O₃, themethod comprising the steps of: selecting one metal element as the firstmetal element Ln from the group consisting of Sc, Y, Ho, Er, Tm, Yb, andLu; and selecting one metal element as the third metal element A fromthe group consisting of Ti, Nb, Ta, and Ru; mixing nitrate of the firstmetal element Ln, manganese nitrate, and oxide of the third metalelement A, so that x falls within the range of 0.005 to 0.2 and y fallswithin the range of 0.9 to 1.0, and thereafter performing primaryfiring; and subjecting a resultant material from the primary firingprocess to grinding and thereafter performing secondary firing at therange of 600 to 1200° C. for 1 to 5 hours.

Further, it is preferable to subject the resultant material from theprimary firing process to grinding and thereafter perform secondaryfiring at 1000° C. for 1 hour.

The oxidation catalyst for cleaning exhaust gas produced according tothe method of the present invention is one in which metal A as a thirdmetal component is added to a composite metal oxide represented by thegeneral formula: LnMnO₃ to produce a distortion in the crystal latticethereof, or the metal component A is added thereto to produce a defectin a portion of the crystal lattice as well as the distortion in thecrystal lattice, thereby increasing the catalytic activity and reducingthe bond energy of oxygen in the crystal lattice. As a result, theabove-mentioned oxidation catalyst for cleaning exhaust gas can oxidizecontents such as particulates and high boiling point hydrocarbonscontained in exhaust gas from internal-combustion engines at a lowertemperature and also cause a faster oxidation than an oxidation catalystcomprising a compound represented by the general formula: LnMnO₃.

The oxidation catalyst for cleaning exhaust gas produced according tothe method of the present invention produces a lower bond energy ofoxygen in the crystal lattice of the composite metal oxide representedby the general formula: LnMnO₃, first when y=1, by metal A as a thirdmetal component being added to the oxide to cause a distortion in thecrystal lattice. As a result, the above-mentioned oxidation catalyst forcleaning exhaust gas can oxidize the particulates, high boiling pointhydrocarbons, and the like at a lower temperature than the oxidationcatalyst comprising the composite metal oxide represented by the generalformula: LnMnO₃. Here, x less than 0.005 renders insufficient the effectof producing a distortion in the crystal lattice; x more than 0.2 makesit difficult to maintain the crystal lattice.

Then, the oxidation catalyst for cleaning exhaust gas produced accordingto the method of the present invention produces a lower bond energy ofoxygen in the crystal lattice of the composite metal oxide representedby the general formula: LnMnO₃, when 0.9≦y<1, by metal A as a thirdmetal component being added to the oxide to cause a defect in a portionof the Ln site constituting the crystal lattice as well as a distortionin the crystal lattice. As a result, the above-mentioned oxidationcatalyst for cleaning exhaust gas can oxidize the particulates, highboiling point hydrocarbons, and the like at a lower temperature and alsocause a faster oxidation than the oxidation catalyst comprising thecomposite metal oxide represented by the general formula: LnMnO₃.

Here, y less than 0.9 produces an excessive defect to make it difficultto maintain the crystal lattice; y at 1 cannot produce a defect in thecrystal lattice. In addition, x can be in the above-described range tobalance the positive and negative electric charges of the constituentatoms in the composite metal oxide.

In the method mentioned above, Ln may be a metal selected from the groupconsisting of Sc, Y, Ho, Er, Tm, Yb, and Lu, but is preferably Y.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the effects of catalysts for cleaning exhaustgas produced in accordance with the method of the present invention;

FIG. 2 is a graph showing the effect of a catalyst for cleaning exhaustgas produced in accordance with the method of the present invention;

FIG. 3 is a graph showing the effects of catalysts for cleaning exhaustgas produced in accordance with the method of the present invention;

FIG. 4 is a graph showing the effects of catalysts for cleaning exhaustgas produced in accordance with the method of the present invention;

FIG. 5 is a graph showing the effect of a catalyst for cleaning exhaustgas produced in accordance with the method of the present invention;

FIG. 6 is a graph showing the effect of a catalyst for cleaning exhaustgas produced in accordance with the method of the present invention;

FIG. 7 is a graph indicating the relationship between the secondaryfiring condition during production procedure of the composite metaloxide and the burning temperature of the carbon black; and

FIG. 8 is a graph indicating the relationship between the secondaryfiring condition during production procedure of the composite metaloxide and the specific surface area of the composite metal oxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will now be described in furtherdetail with reference to the accompanying drawings.

A catalyst for cleaning exhaust gas according to a first aspect of thepresent embodiment comprises a composite metal oxide represented by thegeneral formula: YMn_(1-x)A_(x)O₃, wherein A is a metal selected fromthe group consisting of Ti, Nb, Ta, and Ru; and 0.005≦x≦0.2. Thecomposite metal oxide produces a lower bond energy of oxygen in thecrystal lattice of YMnO₃ by a portion of Mn being the metal A to cause adistortion in the crystal lattice. As a result, the metal oxide can havean increased catalytic activity compared to YMnO₃ and can oxidizecontents such as particulates and high boiling point hydrocarbonscontained in the exhaust gas at a lower temperature.

A catalyst for cleaning exhaust gas according to a second aspect of thepresent embodiment is a composite metal oxide similar to the catalystfor cleaning exhaust gas according to the first aspect, but differentfrom it only in that the former is represented by the general formula:Y_(y)Mn_(1-x)A_(x)O₃ except with 0.9≦y<1. The composite metal oxideproduces a lower bond energy of oxygen in the crystal lattice of YMnO₃by a defect arising in a portion of the Y site constituting the crystallattice as well as by a portion of Mn being the metal A to cause adistortion in the crystal lattice. As a result, the metal oxide can havean increased catalytic activity compared to YMnO₃, can oxidize contentssuch as particulates and high boiling point hydrocarbons contained inthe exhaust gas at a lower temperature, and even can promote theoxidation.

Here, the above-described x and y are set so as to balance the positiveand negative electric charges of the constituent atoms in the compositemetal oxide.

By way of example, Y and Mn are positive trivalent, and O is negativebivalent. Accordingly, when the metal A is one of Ti and Ru and positivetetravalent, setting y=1−x/3 and x=0.15 leads to the general formula:Y_(y)Mn_(1-x)A_(x)O₃ being equal to Y_(0.95)Mn_(0.85)A_(0.15)O₃.

Here, the sum of positive charges is:(3×0.95)+(3×0.85)+(4×0.15)=2.85+2.55+0.60=+6.00; andthe sum of negative charges is:(−2)×3=−6.

Therefore, it follows that (+6.00)+(−6)=0, the positive and negativecharges being balanced.

When the metal A is one of Nb and Ta and positive pentavalent, settingy=1−2x/3 and x=0.075 leads to the general formula: Y_(y)Mn_(1-x)A_(x)O₃being equal to Y_(0.95)Mn_(0.925)A_(0.075)O₃.

Here, the sum of positive charges is:(3×0.95)+(3×0.925)+(5×0.075)=2.85+2.775+0.375=+6.00; andthe sum of negative charges is:(−2)×3=−6.

Therefore, it follows that (+6.00)+(−6)=0, the positive and negativecharges being balanced.

When the metal A is one of Nb and Ta and positive pentavalent, y=1−2x/3and x=0.15 may be also set.

This setting leads to the general formula: Y_(y)Mn_(1-x)A_(x)O₃ beingequal to Y_(0.9)Mn_(0.85)A_(0.15)O₃.

Here, the sum of positive charges is:(3×0.9)+(3×0.85)+(5×0.075)=2.7+2.55+0.75=+6.00; andthe sum of negative charges is:(−2)×3=−6.

Therefore, it follows that (+6.00)+(−6)=0, the positive and negativecharges being balanced.

Examples and Comparative Example of the present invention will now begiven.

EXAMPLE 1

In this Example, first, yttrium acetate, manganese nitrate, and anatasetype titanium oxide were used in such amounts that a molar ratio thereofof 1:0.95:0.05 is obtained, and mixed in a ball mill for 5 hours,followed by primary firing at 250° C. for 30 minutes, at 300° C. for 30minutes, and at 350° C. for one hour. Ethanol was then added to theresultant material from the primary firing process, which was thensubjected to wet grinding using a ball mill before drying, followed bysecondary firing at 1,000° C. for one hour to provide a powder of thecomposite metal oxide represented by YMn_(0.95)Ti_(0.05)O₃.

The composite metal oxide powder obtained in this Example was thensubjected to differential thermal analysis (DTA) for the activityevaluation thereof. The differential thermal analysis was performed byusing the composite metal oxide powder obtained in this Example as acatalyst for cleaning exhaust gas to mix 2.5 mg of carbon black with 50mg of the catalyst, followed by heating the mixture at a rate oftemperature rise of 10° C./minute under an atmosphere of an air streamof 15 ml/minute to determine a relationship between heat flow andtemperature.

The above-described carbon black corresponds to particulates or a highboiling point hydrocarbon contained in the exhaust gas. In the heatflow, the peak thereof indicates the burning temperature of the carbonblack; a higher peak shows that the burning is more promoted. The resultis shown in FIG. 1.

EXAMPLE 2

In this Example, the composite metal oxide represented byYMn_(0.95)Nb_(0.05)O₃ was obtained just as described in Example 1 exceptfor the use of niobium oxide in place of anatase type titanium oxide.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown in FIG. 1.

EXAMPLE 3

In this Example, the composite metal oxide represented byYMn_(0.95)Ta_(0.05)O₃ was obtained just as described in Example 1 exceptfor the use of tantalum oxide in place of anatase type titanium oxide.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown in FIG. 1.

EXAMPLE 4

In this Example, the composite metal oxide represented byYMn_(0.95)Ru_(0.05)O₃ was obtained just as described in Example 1 exceptfor the use of ruthenium oxide in place of anatase type titanium oxide.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown in FIG. 1.

COMPARATIVE EXAMPLE 1

In this Comparative Example, the composite metal oxide represented byYMnO₃ was obtained just as described in Example 1 except for no use ofanatase type titanium oxide.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Comparative Example as a catalyst for cleaningexhaust gas. The result is shown in FIG. 1.

It is apparent from FIG. 1 that the catalysts for cleaning exhaust gasof Examples 1 to 4 can oxidize (burn) the above-described carbon blackat low temperature compared to the catalyst for cleaning exhaust gas ofComparative Example 1, which comprises the composite metal oxiderepresented by YMnO₃.

EXAMPLE 5

In this Example, the composite metal oxide represented byY_(0.95)Mn_(0.85)Ti_(0.15)O₃ was obtained just as described in Example 1except for the use of yttrium acetate, manganese nitrate, and anatasetype titanium oxide in such amounts that a molar ratio thereof of0.95:0.85:0.15 is obtained.

EXAMPLE 5-2

In this example, the composite metal oxide represented byY_(0.95)Mn_(0.85)Ti_(0.15)O₃ was obtained just as described in Example 5except that the secondary firing was conducted at 600° C. for 5 hours.

EXAMPLE 5-3

In this example, the composite metal oxide represented byY_(0.95)Mn_(0.85)Ti_(0.15)O₃ was obtained just as described in Example 5except that the secondary firing was conducted at 1200° C. for 1 hour.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown together with the result of Comparative Example 1 inFIG. 2.

It is apparent from FIG. 2 that the catalyst for cleaning exhaust gas ofExample 5 can oxidize (burn) the above-described carbon black at lowtemperature and can achieve the effect of further promoting theoxidation, compared to the catalyst for cleaning exhaust gas ofComparative Example 1, which comprises the composite metal oxiderepresented by YMnO₃.

EXAMPLE 6

In this Example, the composite metal oxide represented byY_(0.95)Mn_(0.925)Nb_(0.075)O₃ was obtained just as described in Example1 except for the use of yttrium acetate, manganese nitrate, and niobiumoxide in such amounts that a molar ratio thereof of 0.95:0.925:0.075 isobtained.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown together with the result of Comparative Example 1 inFIG. 3.

EXAMPLE 7

In this Example, the composite metal oxide represented byY_(0.9)Mn_(0.85)Nb_(0.15)O₃ was obtained just as described in Example 1except for the use of yttrium acetate, manganese nitrate, and niobiumoxide in such amounts that a molar ratio thereof of 0.9:0.85:0.15 isobtained.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown together with the result of Comparative Example 1 inFIG. 3.

It is apparent from FIG. 3 that the catalysts for cleaning exhaust gasof Examples 6 and 7 can oxidize (burn) the above-described carbon blackat low temperature and can achieve the effect of further promoting theoxidation, compared to the catalyst for cleaning exhaust gas ofComparative Example 1, which comprises the composite metal oxiderepresented by YMnO₃.

EXAMPLE 8

In this Example, the composite metal oxide represented byY_(0.95)Mn_(0.925)Ta_(0.075)O₃ was obtained just as described in Example1 except for the use of yttrium acetate, manganese nitrate, and tantalumoxide in such amounts that a molar ratio thereof of 0.95:0.925:0.075 isobtained.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown together with the result of Comparative Example 1 inFIG. 4.

EXAMPLE 9

In this Example, the composite metal oxide represented byY_(0.9)Mn_(0.85)Ta_(0.15)O₃ was obtained just as described in Example 1except for the use of yttrium acetate, manganese nitrate, and tantalumoxide in such amounts that a molar ratio thereof of 0.9:0.85:0.15 isobtained.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown together with the result of Comparative Example 1 inFIG. 4.

It is apparent from FIG. 4 that the catalysts for cleaning exhaust gasof Examples 8 and 9 can oxidize (burn) the above-described carbon blackat low temperature and can achieve the effect of further promoting theoxidation, compared to the catalyst for cleaning exhaust gas ofComparative Example 1, which comprises the composite metal oxiderepresented by YMnO₃.

EXAMPLE 10

In this Example, the composite metal oxide represented byY_(0.95)Mn_(0.85)Ru_(0.15)O₃ was obtained just as described in Example 1except for the use of yttrium acetate, manganese nitrate, and rutheniumoxide in such amounts that a molar ratio thereof of 0.95:0.85:0.15 isobtained.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown together with the result of Comparative Example 1 inFIG. 5.

It is apparent from FIG. 5 that the catalyst for cleaning exhaust gas ofExample 10 can oxidize (burn) the above-described carbon black at lowtemperature and can achieve the effect of further promoting theoxidation, compared to the catalyst for cleaning exhaust gas ofComparative Example 1, which comprises the composite metal oxiderepresented by YMnO₃.

EXAMPLE 11

In this Example, the composite metal oxide represented byYMn_(0.995)Ru_(0.005)O₃ was obtained just as described in Example 1except for the use of yttrium acetate, manganese nitrate, and rutheniumoxide in such amounts that a molar ratio thereof of 1:0.995:0.005 isobtained.

A relationship between heat flow and temperature was then determinedjust as described in Example 1 except for the use of the composite metaloxide obtained in this Example as a catalyst for cleaning exhaust gas.The result is shown together with the result of Comparative Example 1 inFIG. 6.

It is apparent from FIG. 6 that the catalyst for cleaning exhaust gas ofExample 11 can oxidize (burn) the above-described carbon black at lowtemperature and can achieve the effect of further promoting theoxidation, compared to the catalyst for cleaning exhaust gas ofComparative Example 1, which comprises the composite metal oxiderepresented by YMnO₃.

COMPARATIVE EXAMPLE 2

In this Comparative Example, the composite metal oxide was obtainedunder the same conditions as those of the composite metal oxides asdescribed in Example 5, Example 5-2 and Example 5-3, respectively,except for the secondary firing condition in which the composite metaloxide is fired not at 1000° C. for 1 hour but at 1200° C. for 10 hours,after the resultant material from the primary firing process wassubjected to grinding.

It is apparent from FIG. 7 that the burning temperatures of the carbonblack of the composite metal oxides according to Example 5, Example 5-2and Example 5-3, respectively, are lower by 30° C. to 100° C. than inthe case of the composite metal oxide according to Comparative Example2. Thus, it is clear that by controlling the secondary firing conditionas in the present invention, it is possible to provide excellentoxidation characteristic to the composite metal oxide.

Further, as is apparent from FIG. 8, the composite metal oxide accordingto Example 5 has larger specific surface area than the composite metaloxide according to Comparative Example 2. Thus, it is clear that bycontrolling the secondary firing condition as in the present invention,it is possible to provide excellent catalyst activity to the compositemetal oxide. Here, the specific surface area may be measured accordingto know technique such as a BET method and the like.

1. A method of producing an oxidation catalyst for cleaning exhaust gas,which oxidizes contents in exhaust gas from internal-combustion enginesto clean the gas, and which comprises a composite metal oxide includinga first metal element Ln, Mn as a second metal element, and a thirdmetal element A, represented by the general formula Ln_(y)Mn_(1-x) A_(x)O₃, the method comprising the steps of: selecting one metal element asthe first metal element Ln from the group consisting of Sc, Y, Ho, Er,Tm, Yb, and Lu; and selecting one metal element as the third metalelement A from the group consisting of Ti, Nb, and Ta; mixing nitrate ofthe first metal element Ln, manganese nitrate, and oxide of the thirdmetal element A, so that x has a value ranging from 0.005 to 0.2, and yhas a value ranging from 0.9 to 1.0, and thereafter performing primaryfiring; and subjecting a resultant material from the primary firingprocess to grinding and thereafter performing secondary firing at therange of 600 to 1200° C. for 1 to 5 hours.
 2. The method according toclaim 1, wherein the resultant material from the primary firing processis subjected to grinding and thereafter secondary firing is performed at1000° C. for 1 hour.
 3. The method according to claim 1, wherein Y isselected as the first metal element Ln.
 4. The method according to claim1, wherein the first metal element Ln is selected from the groupconsisting of Sc, Ho, Er, Tm, Yb, and Lu.
 5. The method according toclaim 1, wherein the resultant material from the primary firing processis subjected to grinding and thereafter secondary firing is performed at1200° C. for 1 hour.
 6. The method according to claim 2, wherein Ti isselected as the third metal element A.
 7. The method according to claim5, wherein Ti is selected as the third metal element A.